The present invention generally relates to a telephone line access apparatus and, in particular, relates to one such line access apparatus including a first stage high voltage protection means, a second stage filter means, and means for housing the first and second stages, the housing means being adapted to plug into a main distribution frame.
In general, the public telephone systems implemented throughout the world include a plurality of subscriber pairs of wires each being fed at one end thereof, into a main distribution frame. From the main distribution frame the subscriber pairs are connected to the telephone company switching equipment. The switching equipment is connected to networking equipment, such as, for example, long line cables, microwave links and the like. The switching equipment performs the services of interconnecting pairs of subscriber wires and the networking equipment establishes telephonic voice communication between different switches to establish connection between different ones of the subscriber pairs. Typically, in order to standardize and enhance network wide equipment utilization, all of the elements of the telephone system have been limited to carrying only a relatively small voice band of frequencies regardless of the actual bandwidth capability of the individual elements. Usually, the band limitation imposed ranges from DC to about 4,000 Hertz. This particular limitation has traditionally been imposed primarily due to the bandwidth capability of the various switch and networking equipment, rather than the capability of the subscriber pairs of wires on the subscriber side of the main distribution frames. As it happens, however, the subscriber pairs of wires are usually copper and, in a vast majority of installations, are quite capable of supporting a much larger bandwidth than the limited allowable bandwidth. Specifically, as part of standardization the subscriber pairs of wires have been limited to carrying signals below 4 kiloHertz. One reason for extending the frequency limitations to all elements of such systems is that, in many instances, a higher frequency signal, if injected onto a subscriber pair, would be disruptive if transmitted through the switch to the networking, or transmission, equipment.
As a result of the relatively recent advances in the transmission and use of digitized information, the subscriber pairs of wires of telephone companies have become the object of interest for communicating many different types of information with a consumer. One major reason for this interest is that such communication paths exist and connect to an extremely large percentage of the populus. The use of such existing wiring would clearly result in massive savings in introducing new services since new and redundant wiring of such a large potential market would be avoided. To date, in order to overcome the bandwidth limitations imposed by the transmission equipment and exploit the available bandwidth of subscriber pairs, equipment manufacturers and system developers have made use of filters that are installed in series between each subscriber pair of wires and the central office switching equipment in order to comply with the signal levels at frequencies above the voice band. Nevertheless, the actual installation and implementation of such digital signal equipment and systems has been severely limited primarily due to the expense involved in rewiring existing main distribution frames in order to install the necessary filters. Consequently, individual and isolated applications, particularly, in the public switching environment are developed or implemented due to the tremendous overhead cost of the administration and installation of such filters. That is, only those services that can justify the cost associated with the filter installation and the costs associated with rewiring a main distribution frame have beem implemented. Further, even when such an application, and the demand therefor, may justify the exorbitant cost for the filters, the actual installation quite frequently becomes extremely difficult because of the crowding of current main distribution frames and the wiring associated therewith.
In any event, the requirements imposed upon such filters with respect to the signals that must be allowed to pass without attenuation, for example, the DC current feed and ring voltages used in voice communications, often make the implementation of the filters large and prohibitively expensive. Because of the relatively large size of the passive series filtering elements they are usually remotely located from the main distribution frame. Therefore, currently, because of remote location and the series nature of the connection of such filters, the main distribution frame nearest the location of the filter has to be rewired with additional pairs of wires. That is, one pair of wires must be provided for connecting to the outside filter from the subscriber pair and another pair of wires must be provided from the filter location back to the main distribution frame for continuing the subscriber pair from the filter to the central office switching equipment. This is sometimes extremely difficult because a large number of main distribution frames are currently operating at greater than 50% capacity.
Nevertheless, passive filter configurations that are capable of passing DC currents and inserted in series with the telephone line for low bandpass and bandstop filtering are known.
Alternatively, active filters that are relatively small are available using modern semiconductor technology. However, the application of these small active filters to subscriber pairs is generally not feasible due to the high feed and ring voltages normally present on telephone lines. Further, such active filters are extremely susceptable to catastrophic failure due to high voltage transients created by high voltage inductions and lightning strikes.
Consequently, a relatively small passive filter apparatus that can be implemented without the presently required rewiring of the main distribution frame is necessary to fully realize the implementation, development and provision of additional services to a customer premise via an auxiliary service provider. Such an apparatus would inherently be comparatively inexpensive and convenient from the telephone administration point of view.